Not enough.  In the US, statistics suggest about 2%.

This world of people that live and work among the hearing majority in every country is currently under-reached with the gospel of Christ.  They do not overhear conversations among others, radio and uncaptioned television broadcasts, or random bits of news and debate.  They do not, in most cases, make seeking God a priority any more than we (the hearing) do.

Think of how many radio stations, churches, book stores and Bible translations we have at the ready, not to mention friends, family, and coworkers to whom we can turn (and we recognize them by fragments of overheard conversation).  Now drop that number by about 80-90% and ask yourself if you would put in the time and effort required to seek a God who society now declares to be a low priority.

Would you continue looking for Him if you enter His house of worship and find it to be a place of strangers, yourself ignored or avoided, and the information a struggle to grasp, because of a language barrier?  When you have all you can handle in juggling family and friends, job and commerce in a society that looks familiar but operates in a foreign language…how much work would you add to your load in order to find and build a relationship with “their” God?

For more info, Deaf Ministry Considerations is a good place to start.